The present invention relates to readout devices for use in ring laser angular rate sensors. The present invention provides a readout device that displays improved performance at low input rates.
Ring laser angular rate sensors or ring laser gyroscopes are fairly well known in the field of angular rate sensing. Examples of ring laser gyroscopes are shown in U.S. Pat. No. 3,373,650 which is assigned to the assignee of the present invention. In summary, two counter propagating light beams are maintained within a closed loop path to form a rotation sensing device. Rotation of this device around an axis which is normal to a plane containing the closed loop path causes the relative path lengths in either direction to change. Detections of these changes in path length can be used to measure the rate of rotation.
A common problem occurs when the angular rate sensors are rotated at a very low rate. If the angular rate sensor is held still in inertial space for a period of time the counter propagating light beams tend to resonate together or "lock-in". once the two beams lock-in to one another it is then not possible to detect differences in relative path length.
In order to avoid lock-in the angular rate sensor is rotationally oscillated or dithered. If the readout signal is averaged over a period of time the average rotation rate of the oscillation signal is zero. Therefore this oscillation or dither will not affect the sensitivity of the gyro. The dither signal or rotational oscillation of the ring laser gyroscope can be removed from the output signal by either geometrically eliminating the affects of this oscillation or electronically removing the dither signal. To eliminate the dithering geometrically the optical elements making up the readout must be appropriately placed. Once appropriately placed the change in path length due to dithering is cancelled out by the scale factor of the gyroscope.
To accomplish geometric removal of the dither signal or "dither stripping" some of the optic elements making up the readout device must not be connected to the laser block itself. Therefore, during dithering the laser block itself is rotationally oscillated while the readout optics are not.
Numerous readouts are traditionally placed on the ring laser angular rate sensor to perform numerous other functions. An example of these functions are laser intensity sensing to assure the optical signals within the ring laser angular rate sensor are resonating at a sufficiently strong level.
Backscattering also effects the performance of the gyro. Specifically, the low-rate linearity is detrimentally affected by backscattering.
One source of lock-in is backscattering. Backscattering occurs when the optical signal propagating in one direction within the ring laser gyro is scattered back into the gyro but in the opposite direction. Backscattering can be due to sources within the laser cavity or sources outside the laser cavity. Since it is desirable that the optical signals within the ring laser gyro oscillate independently at their particular frequencies, backscatter is detrimental as it causes an unwanted coupling influence between the two optical signals. Gyro dither is effective in mitigating the effect of scatter sources that oscillate along with the gyro block, however dithering is not effective in mitigating the effects of case mounted components. There are numerous sources of back scattering including the readout optics used, optical sensors attached to the laser gyro or the gyro case, and even the laser gyro case itself.
Referring now to FIG. 1 there is shown a prior art readout system. A laser block 10 contains a number of tunnels or bores 12 which create a closed loop path within laser block 10. Bores 12 carry the optical signals which make up the angular rate sensor. Upon one corner of laser block 10 there is an output mirror 14 which is partially reflective and partially transmissive. Output mirror 14 allows a portion of the optical signals within laser block 10 to be transmitted through output mirror 14 while another portion of the optical signals is reflected back into the adjoining bore 12. Shown as lines on FIG. 1 are a counter clockwise signal or CCW signal 20 and a clockwise signal or cw signal 22. While FIG. 1 represents only one corner of the ring laser gyro it is understood that the gyro block 10 makes up a closed loop path with mirrors on the corners of the closed loop path. CCW signal 20 is transmitted through output mirror 14 and toward a corner cube 16 which is mounted to the gyro case (not shown). Corner cube 16 causes CCW signal 20 to be translated and reflected back towards output mirror 14. Upon meeting an upper surface 24 of output mirror 14 CCW signal 20 is reflected off surface 24 and directed onto an optical sensor 18. Meanwhile, CW signal 22 is transmitted through output mirror 14 and directly onto optical sensor 18. Optical sensor 18 receives a combination of CCW signal 20 and CW signal 22. Those skilled in the art will recognize that this combination of these two signals can be used to detect rotation.
In summary, the effects of dithering are cancelled out by the scale factor of the gyro. Dithering causes a change in the path lengths of the optical signals. This change in path length is then designed to be exactly cancelled out by the gyro scale factor. However, changes in path length due to common rotation of both the gyro block 10 and the gyro case will still be detected.